Add note on supported Python versions to readme.
Add note on supported Python versions to readme.

andrewsomething/libcloud,Kami/libcloud,apache/libcloud,mistio/libcloud,mistio/libcloud,Kami/libcloud,andrewsomething/libcloud,mistio/libcloud,apache/libcloud,andrewsomething/libcloud,apache/libcloud,Kami/libcloud